Rewrite exchange heap in Rust

[brson]

The exchange heap is currently written in C++.

[brson]

Blocked on #4812

[brson]

I have this partially done in a branch, but it requires both Rust and C++ components because of the above blocker.

[brson]

Not 0.7

[thestinger]

Closing in favour of #5323. There's only one call left to reserve_vec_exact in an extra::time helper, and that uses the C++ exchange allocator.